Air pollution is a very serious and urgent international problem. The sources of air pollution are primarily the products of combustion and are numerous and widespread.
Many of the air pollutants are in the form of sulfur-bearing flue gases discharged by fossil-fuel-burning electrical power generating plants or other industries. While the precise impact of these pollutants on the environment is still a subject of speculation, there nonetheless is considered to be a possible negative effect. Yet, under foreseeable circumstances, it will be necessary to burn more and more fuel to meet the demands of a rapidly growing population requiring for each person evermore heating comfort and power, and the fuel which will generally be used will not contain much less sulfur, but will likely contain more sulfur.
Thus, sulfur oxides, principally present as sulfur dioxide, are found in the waste gases discharged from many metal refining and chemical plants, and in the flue gases from power plants generating electricity by the combustion of fossil fuels. In addition, sulfur-containing gases, notably sulfur dioxide, may be formed in the partial combustion or gasification of sulfur-containing fuels, such as coal or petroleum residua. The control of air pollution resulting from the discharge of sulfur dioxide into the atmosphere has thus become increasingly urgent.
The most common flue gas desulfurization (FGD) process is known as the "wet process". In that process the sulfur dioxide-containing flue gas is scrubbed with a slurry containing, e.g., limestone. The scrubbing takes place, for example, in an absorption tower in which the gas flow is countercurrent to and in intimate contact with a stream of slurry. The slurry may flow over packing or trays, or be sprayed into an open section of the tower. The spent slurry product of this FGD process contains both calcium sulfite and calcium sulfate. It has been found to be advantageous to convert the calcium sulfite in the product to calcium sulfate by bubbling air or other oxygen-containing gas through the slurry.
Gypsum has many advantages, such that it is much in demand, essentially harmless, incombustible, and chemically stable, and it can be disposed of as waste material in land reclamation without the danger of secondary public nuisance. Moreover, limestone can be used as a neutralizing agent in desulfurization with gypsum as a by-product. The former is not only exceptionally cheap as compared with other neutralizing agents, but it is also readily available in a relatively long-lasting stable form.
One system for the desulfurization of flue gases by means of a limestone-containing scrubbing liquid involves a double loop or circuit for the liquid streams being employed, such as shown in Biedell et al, U.S. Pat. No. 4,351,804. In that system, the gas to be treated first enters a "quencher" and then passes to an "absorber", and the two liquid loops or circuits are each connected to one of these two units and to each other. A slurry containing limestone and gypsum solids fed to the quencher is contacted with the flue gas being treated and, after contact, accumulates at the bottom of the quencher, and air is introduced into the liquid accumulated in the quencher to oxidize the calcium sulfite which has been formed from the reaction between SO.sub.2 and limestone, to calcium sulfate (gypsum).
Limestone is fed to the absorber feed tank from which a slurry containing limestone, calcium sulfite and calcium sulfate is fed to the absorber for contact with the flue gas. This slurry is recirculated to the feed tank. A portion of the slurry in the absorber feed tank overflows to the quencher and a portion is sent to a set of hydroclones. The hydroclones separate the solids from the liquid in the slurry and the concentrated solids are discharged directly to the quencher. The dilute stream from the hydroclones is returned to the absorber feed tank. The purpose of this step is to control the suspended solids concentration in the absorber feed tank slurry.
Objectives of a forced oxidized limestone desulfurization process are to maximize the purity of the gypsum produced since the commercial attractiveness of the gypsum is a function of its purity, and to produce a low degree of sulfur gas (expressed as SO.sub.2) in the flue gas effluent at a reasonable gas flow.
While the above-mentioned double loop system for desulfurization utilizing limestone as a reagent is generally effective, difficulty has been experienced in meeting the stated objectives of a high degree of desulfurization with simultaneous production of gypsum of high purity. Difficulty has also been experienced by reason of gypsum scale formation in the treating apparatus associated with the absorber. The chemical scale which grows on the absorber packing eventually accumulates until it obstructs the normal path of the flue gas through the absorber. At that time, costly maintenance must be performed on the absorber to remove the scale.